The formation of foam in lubricating oils during use is undesirable because the presence of foam dramatically reduces the effectiveness of the lubricating oil, interfering with the ability of the lubricating oil to wet the surface being lubricated and interfering with the ability of the lubricating oil to carry off wear products, decomposition products, sludge, soot, and dissipate heat in the equipment being lubricated.
Foam formation in lubricating oils is counteracted by the use of defoamant. Defoamants function in part by being insoluble in the lubricant. The very characteristic which makes a material useful as a defoamant also has the negative effect of limiting the amount of defoamant that can be added to the lubricant to form a stable mixture, i.e., a mixture in which the defoamant remains in solution with the lubricant and does not separate out of the lubricant oil and therefore become ineffective for its intended purpose.
Numerous defoamants are known in the industry and have been employed with varying degrees of success. Defoamants include silicone anti-foam agents (or defoamants) such as polydimethylsiloxane (PDMS) oils and polymers thereof, silicone glycols, fluorinated PDMS and polyacrylate esters. Defoamants typically have kinematic viscosities at 25° C. in the range of from about 352 mm2/s to about 120,000 mm2/s and even higher.